What are the real-life applications of transfer learning?
One application I know of being used in industry is of image classification, by only training the last layer of one of the inception models released by Google,  with the desired number of classes. I can't provide specific details. 

Transfer learning is useful when:


You do not have the resources (time, processing power, etc.) to train a DL  model from scratch.
You can compromise a bit on accuracy.